ast.FormattedValue.format_spec unnecessarily wrapped in JoinedStr

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-02-26.12:43:02.953>
labels = ['type-bug', 'library', '3.9']
title = 'ast.FormattedValue.format_spec unnecessarily wrapped in JoinedStr'
updated_at = <Date 2020-03-04.19:58:04.173>
user = 'https://github.com/ikamensh'

bugs.python.org fields:

activity = <Date 2020-03-04.19:58:04.173>
actor = 'BTaskaya'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-02-26.12:43:02.953>
creator = 'Ilya Kamenshchikov'
dependencies = []
files = []
hgrepos = []
issue_num = 39760
keywords = []
message_count = 4.0
messages = ['362691', '362696', '362699', '362741']
nosy_count = 4.0
nosy_names = ['eric.smith', 'serhiy.storchaka', 'Ilya Kamenshchikov', 'BTaskaya']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39760'
versions = ['Python 3.9']